A METHOD FOR IMPROVEMENT OF SAFETY FEATUR ES OF LARGE FAST BREEDER-REACTORS - NUMERICAL-SIMULATION OF UNPROTECTED LOSS-OF-FLOW ACCIDENT IN AN LMFBR EQUIPPED WITH GAS-EXPANSION MODULES

Numerical simulation of an unprotected loss-of-flow (ULOF) accident ha s been performed for a large liquid-metal-cooled fast breeder reactor (LMFBR) equipped with gas expansion modules (GEMs) in the radial perip hery of the reactor core. The effectiveness of the GEMs in small fast reactors was demonstrated already in the passive safety testing in the Fast Flux Test Facility. According to neutronic calculations based on the transport theory, even in large reactors of electrical power 600 to 1,300 MW, the reactivity worth of GEMs, which replace one layer of radial blanket fuel subassemblies, ranges from -1.9$ to -1.4$, dependi ng on the size of the core. A simulation of ULOF transient was perform ed with a 5.5s flow-halving time in a 600 MWe LMFBR equipped with GEMs of -1.9$ reactivity worth. The result showed that, if 10% of the rate d core coolant flow by pony motors was available following the main pu mp coastdown, the GEM reactivity alone could bring the reactor subcrit ical and the predicted maximum coolant temperature was substantially l ower than the sodium boiling point. The reactivity worth calculations, a modeling of gas expansion behavior, and ULOF simulation together wi th needs of further development for the GEM application are described.